Electronic drawings that give comprehensive details of how to build and test equipment essential for making nuclear bombs have vanished and could be put up for sale on the international black market, according to UN investigators.

The blueprints, running to hundreds of pages, show how to make centrifuges for enriching uranium. In addition, the investigators have been unable to trace key components for uranium centrifuge rigs and fear that drawings for a nuclear warhead have been secreted away and could be for sale.

Inspectors at the UN's nuclear authority, the International Atomic Energy Agency, have been investigating the worst nuclear smuggling racket ever uncovered, headed by the Pakistani scientist Abdul Qadeer Khan. The operation was discovered two years ago to be selling sensitive nuclear technology to Libya and Iran.

A senior official said several sets of blueprints for uranium centrifuges - the so-called P-1 and more advanced P-2 systems which were peddled by the Khan network - have gone missing.

"We know there were several sets of them prepared," said the official. "So who got those electronic drawings? We have only actually got to the one full set from Libya. So who got the rest, the copies?

"We have no evidence they were destroyed. One possibility is another client. We just don't know where they are."

A European diplomat privy to western intelligence on the Khan network added: "This is what keeps people awake at night. It's very sensitive. The fact that there are [nuclear] proliferation manuals kicking around is deeply disturbing."

The blueprints detail how to manufacture the components for a uranium centrifuge, what materials are needed, how to assemble the machines, and how to test them. The centrifuges are the main route to producing bomb-grade uranium. Uranium concentrate is converted into uranium hexafluoride gas which can be spun through cascades of centrifuges at super-high speeds to be enriched to weapons grade.

"The big question is who else got this stuff [apart from Iran and Libya]," the European diplomat said.

Another diplomat pointed out that the Khan network was based in the Middle East and that Khan was known as the father of the Islamic bomb. He suggested that Syria and Egypt could be potential customers for the materials if they were still being offered.

Khan is a national hero for creating the Pakistani nuclear bomb but is under house arrest in Islamabad since confessing to heading the network and being pardoned in February last year.

Although the network's operations extended to Europe, Africa, the Middle East, and the far east, its headquarters were in Dubai. Khan maintained a luxury apartment in Dubai.

Following the uncovering of the network in October 2003, investigators went to the Dubai apartment only to find that it had been emptied, apparently by Khan's daughter Dina.

The Libyan leader, Colonel Muammar Gadafy, confessed to his secret nuclear bomb programme and gave it up in December 2003. Three months later in Tripoli, the UN inspectors were given two CD-roms and one computer hard drive. One CD contained aset of drawings and manuals for the P-1 centrifuge system, the other for the more advanced P-2.

The instructions are in English, Dutch and German, and the designs are from Urenco, the Dutch-British-German consortium which is a leader in centrifuge technology and is the source of Khan's knowhow from his time working there in the 1970s. The CDs and hard drive are at IAEA headquarters in Vienna, where they have been analysed. The investigators now know that the scanning of the original blueprints was done in Dubai and when.

In addition to these blueprints, Khan also supplied Libya with drawings for an old Chinese nuclear warhead design. The drawings, now in Washington under IAEA seal, were not complete, say sources, but were adequate to construct a crude nuclear device.

Investigators suspect that the warhead design was also copied into electronic form and is still available to prospective clients.

"There is reason to believe that there might even be some drawings related to nuclear weaponisation in electronic form," said the senior official.

It is now also clear that multiple components secretly made for Libya's $100m (£54.6m) centrifuge programme did not reach Libya and have gone missing.

From their investigations of the nuclear programmes in Libya and Iran, the IAEA has concluded that pieces of the nuclear jigsaw have not been located.

"We are still missing something from the picture in terms of critical equipment, certain parts of centrifuges ... There is equipment missing important enough for us to search, an amount that makes us worried," said the official.

Around a dozen individuals, including engineers, businessmen, and middlemen, were key figures in the Khan network, with dozens of other companies operating at a secondary level, according to those familiar with the investigation.

Alleged Khan associates have been arrested in the Netherlands, Germany, Switzerland, South Africa, Dubai, and Malaysia, although none of those cases has yet come to full trial. British customs is also conducting an investigation into a British suspect.